U.S. Pat. No. 4,378,769 discloses an ignition circuit for which the supply voltage of a microprocessor is taken off of a capacitor. The capacitor is connected in series with the following: a rectifying diode, a charging resistor limiting the charging current and a charging coil. A voltage is induced in the charging coil by the magnetic field of a rotating magnet wheel. This voltage is dependent upon the rpm of the rotating magnet wheel. The energy content of an induced voltage pulse is low for low rpm and is very high at high rpm. The rpm ranges for two-stroke engines in portable handheld work apparatus lie between 1,000 to 15,000 rpm and in some cases up to 20,000 rpm.
The voltage supply of a microprocessor must be ensured at low rpm as well as at high rpm. Overvoltages can occur at high rpms and a Zener diode is connected in parallel to the capacitor for protection against overvoltages. The avalanche or breakdown voltage of a Zener diode corresponds approximately to the maximum permissible supply voltage. If the supply voltage taken off on the capacitor increases beyond the breakdown voltage at high rpm, then the Zener diode becomes abruptly conductive so that the capacitor is shunted and the overvoltage can decay. At high rpm, an energy pulse is often sufficient to charge the capacitor to the necessary supply voltage for several rotations of the crankshaft. Subsequent energy pulses must therefore be diverted via the Zener diode. A substantial loss of energy occurs which requires a corresponding dimensioning and cooling of the components.